Such processing machines are used notably in the printing and packaging industry, for example, for making cardboard boxes from plate-like elements, such as pre-printed cardboard sheets. In a feeder station, these sheets are taken from a stack located upstream of the machine and are then placed in gripper bars mounted at regular intervals between two lines of chains. The gripper bars and chains makes it possible to convey the sheets into the various subsequent processing stations of the machine. Typically, such stations are devoted to the punching of the sheets, to the ejection of the punching wastes and to the reception in a stack of these punched sheets.
In a paced flow, the lines of chains move and stop periodically so that, during each movement, all the gripper bars engaged with a sheet are moved from one station to the adjacent downstream station. To obtain a quality printing or converting operation, the placement of the sheets within the various successive stations is crucial. In punching a printed sheet, sheet placement in the punching station must be accurate. Specifically, care should be taken that tools used for the punching, for example the punching form of a platen press, are in perfect register with the printing that has been previously done on the sheet.
The document EP 1,044,908 relates to a device and a method for placing plate-like elements in a feeder station. In this method are applied the successive steps consisting in, during the advancement of each plate-like element, activating the gripping element in order to grasp the plate-like element, then measuring the longitudinal placement error, the transverse placement error and the angular placement error of the plate-like element attached to the feeder, relative to a theoretical position, by detecting register marks printed on the plate-like element by first sensors, and finally controlling the gripping element according to the placement errors of the plate-like element to which it is attached.
The device and the method described in this document operates remarkably well and has made it possible to considerably increase the production rates of the processing machines by carrying out the measurements on the fly and the corrections of placement of each plate-like element, without the necessity to stop the plate-like element. Nevertheless, when a plate-like element is very much advanced or when it is very askew, the gripping element may hold the plate-like element on a printed portion instead of the front waste section. There is a risk of damaging the print and the structure of the plate-like element in an area outside the front waste section.
The document WO2011/009567 discloses an improved processing machine, comprising two additional second sensors, placed upstream of the first sensors. In a first step, the two additional second sensors are capable of detecting the passage of a transverse edge of the plate-like element, when the latter is moving, but before it is seized on the fly by the gripping element. Thanks to the measurements of the two second sensors, the position of the gripping element is pre-corrected in order to be well positioned in parallel to the front transverse edge of the plate-like element before grasping it. In a second step, the longitudinal, transverse and lateral placement errors of the plate-like element grasped by the gripping element are measured by the first sensors, by detecting register marks printed on the plate-like element. The gripping element is then controlled according to the placement errors of the plate-like element to which it is attached. The risk of damaging the print and the structure of the plate-like element in an area outside the front waste section can thus be avoided. This method makes it possible to correct placement errors that are more serious and therefore to reduce the risk of machine stoppage associated with an out-of-tolerance placement error of a plate-like element. Generally, the method makes it possible to recover advance or delay of most of the plate-like elements without machine stoppage.
However, it is still not possible to recover a very large delay of the plate-like element, typically when the shift of the plate-like element is higher than 6 mm with respect to the theoretical position. In this case, the plate-like element edge is detected too late and it cannot be rectified. Indeed, with machine speeds in the order of 12,000 sheets/hour, although a theoretical trajectory can be estimated to control the gripping element to bring the plate-like element in time, the accelerations needed to achieve this are too important and cannot be implemented. Such accelerations would involve too important vibrations of the gripping element that could not be stopped in an accurate position, in particular due to the masses that have to be moved and because of the very high precision that is required.
A simple solution to reduce the accelerations of the gripping element could be to anticipate its movements by simply moving the additional second sensors at a most upstream location. However, the plate-like elements that arrive in the gripping element with an important advance could not be detected by these most upstream positioned second sensors. Indeed, the front transverse edge of the advance plate-like element will be covered by the plate-like element located upstream, already taken by the gripper bar and just leaving the place. The front transverse edge of the plate-like element will thus be hidden by the plate-like element located upstream when the second sensors will try to detect it.
Another simple solution could be to detect the passage of the rear edge of the plate-like element as it would not be hidden by the preceding sheet. The machine would thus be informed soon enough to trigger the start of the gripping element in advance, allowing limiting the needed accelerations for catching up the delay. This may be suitable for plate-like elements of high thicknesses, approximately greater than four or five millimeters. Indeed, sensors commercially available are able to detect variations of thicknesses that are representative of the passage of a sheet. However, they are not able to detect smaller thickness with sufficiently accuracy or they are too expensive.